Also in favour of completely silent house collapse - that'd be a meaningfully 'organic' injection of activity to shard-wide life.
But I think activity requirements to refresh are the real key here:
- Allow a 3-6 month buffer to handle kids and holidays
- Allow all kinds of activity to refill the buffer
- Allow all characters on an account to put progress toward that account's house's activity progress
- Scale the activity required with the cost of the deed (not overly punitively: maybe a castle needs 5x more activity than an 8x8)
- Spend proportionally out of the buffer for the number of days added in a sign refresh
So Lois M. McMillionaire has to pay 100 points per month toward sign refreshes to keep her castle up, but she's banked 500+ of her max 600 points by being active. So she goes a week between refreshes, and when she does refresh, she pays 25 points (7 days / 28 days) from her buffer.
And Dave L. Nubenstein must pay only 20 points per month for his 8x8, which (relatively speaking) is about as difficult for him: as a new player his farming speed is pretty low. His max is only 120 (monthly upkeep * 6) but he usually hovers around 30 with his activity level.
Quantifying activity is the tricky bit. Certainly you can have all society progress count, but then you need to account for people who RP, who decorate, who PK, and so on. But after you've figured that out, the system could be pretty solid. (You could even just use time logged in as a rudimentary but probably still somewhat effective metric, because then someone is at least running a client, which represents some logistical overhead. And clients already get logged out if they're inactive.)
You set the activity level requirement low, so the real punishment is really toward the people sitting completely passively on houses. Then at least all those accounts need characters with skills which play (or sit ingame, at least). At the moment, the cost to them (of completely pricing new players out of the housing market) is 20 seconds to do a sign refresh every 35 days, which is way too low.
Effectively this ties house size to activity level. For the land tied up by enormous plots, I think this is perhaps a sensible direction. You can still be barely active and show off your pixels: get a smaller house with a tiny activity-upkeep cost. But if you want to sit on 29x29 of that good good land, you have to at least play 2-3 days a month (or be logged in 2-3 days a month).